Arguments against large cardinals

Question

I started to learn about large cardinals a while ago, and I read that the existence, and even the consistency of the existence of an inaccessible cardinal, i.e. a limit cardinal which is additionally regular, is unprovable in ZFC. Nevertheless large cardinals were studied extensively in the last century and (apart from attempts that went too far as the Reinhardt-Cardinals) nobody ever found a contradiction to ZFC. As a consequence it seems to me that set theorists today don't consider the possibility of the non-existence of a large cardinal. Therefore my questions:

Why is it so unreasonable to think that the existence of large cardinals contradicts ZFC? Are there any mathematicians that do believe that large cardinals don't exist? And what are their arguments?

EDIT: I want to say thank you for all your very interesting answers and comments. It will take some time for me to fully understand them though, however I feel like I have already learned a lot due to this discussion. Thank you!

Answer 1

I do not know of any active set theorists who think large cardinals are inconsistent. At least, within the realm of cardinals we have seriously studied.

[Reinhardt suggested an ultimate axiom of the form "there is a non-trivial elementary embedding $j:V\to V$". Though some serious set theorists found it of possible interest immediately following its formulation, Kunen quickly afterward showed that this is inconsistent, using choice. It is not known whether choice is needed, but current research suggest that, even without choice, natural strengthenings of this axiom may be inconsistent in $\mathsf{ZF}$ alone. This is hardly an argument against large cardinals in general. Instead, it provides us with natural limitations on their reach. For another example, see here.]

There are several active set theorists who do not commit themselves one way or the other to the consistency of large cardinals, but use them if necessary, and do not object on mathematical grounds to arguments that involve them. For some of them, set theory is about all possible (natural) extensions of $\mathsf{ZFC}$, and there are certainly many interesting such extensions (such as $V=L$) that rule out large cardinals. Thus, it is not that they consider the cardinals inconsistent.

(I confess, this may be ignorance on my part.)

And I know of only two mathematicians who years ago were serious set theorists and who have expressed doubts about the consistency of (certain) large cardinals. Neither is currently active within the field, and so their position should be taken with a grain of salt, since it missed the significant results from the late 80s that could very well have forced them to reconsider.

Why do we expect $\mathsf{ZFC}$ to be consistent, to begin with? We expect more than mere consistency, of course, but doubting large cardinals usually means distrust in set theory as a whole. I am not a philosopher, so I will not discuss philosophical positions or justifications. A good reference for the heuristics behind the basic ZFC axioms is the wonderful paper

Penelope Maddy. Believing the axioms. I, J. Symbolic Logic, 53 (2), (1988), 481-511. MR0947855 (89i:03007).

Large cardinals are discussed in its follow-up,

Penelope Maddy. Believing the axioms. II, J. Symbolic Logic, 53 (3), (1988), no. 736-764. MR0960996 (89m:03007).

Maddy's two books on Platonism and Naturalism discuss extensively why the view of a set theoretic universe with large cardinals rather than not is the reasonable choice, given our current understanding, see

Penelope Maddy. Realism in mathematics. The Clarendon Press, Oxford University Press, New York, 1990. MR1075998 (92h:00007).

and

Penelope Maddy. Naturalism in mathematics. The Clarendon Press, Oxford University Press, New York, 1997. MR1699270 (2000e:00009).

The books present several subtle technical points that can only be completely understood once one is aware of the deep connections between large cardinals and (generic) absoluteness. Maddy's more recent views on the subject can be seen here:

Penelope Maddy. Defending the axioms: on the philosophical foundations of set theory. Oxford University Press, Oxford, 2011. MR2779203.

How do set theorists measure the internal plausibility of large cardinal assumptions, beyond their usefulness in proving results? The point of the inner model program (and of its most recent offspring, descriptive inner model theory) is to develop fine structural ("$L$ like") models for large cardinals. These models are canonical in several precise ways, and have a rich internal structure that many set theorists take as evidence of the consistency of the large cardinals under consideration. Thanks to its advances, we have a much clearer view of the set theoretic universe nowadays (for example, we now have the different covering lemmas, and several generic invariance results) than when the program began, motivated by what we now call Gödel's program.

The program has currently reached well past Woodin cardinals, but is not yet at the level of supercompact cardinals. This can be interpreted as saying that, using the strongest tools currently at our disposal, we are fairly certain of the consistency of, say "there is a Woodin limit of Woodin cardinals". Time will tell whether the program will reach supercompactness. If it does not, this will provide us with strong evidence of their inconsistency, though I am not sure anybody actually expects this to be the outcome.

John Steel and Tony Martin have over the years refined something they call "the speech", where they explain their position towards large cardinals. It is well worth reading, and trying to summarize it in a few lines would be an injustice. It can be found in these two postings to the Foundations of Mathematics (FOM) list: 1, 2 (the notation here is $P_T =$ set of $\Pi^0_1$ consequences of $T$), and in the papers from the "Does mathematics need new axioms?" panel, see

Solomon Feferman, Harvey M. Friedman, Penelope Maddy, and John R. Steel. Does mathematics need new axioms?, Bull. Symbolic Logic, 6 (4),(2000), 401–446. MR1814122 (2002a:03007).

Steel's own views are also presented in some detail in Maddy's books. For very recent developments, see his talk:

John Steel. Gödel's program, given at the CSLI meeting at Stanford, June 1, 2013.

At the risk of not being balanced, let me point out some highlights: We have a coherent picture of the universe of sets, with large cardinals. We can, within this picture, interpret theories where there are no such cardinals. However, we do not have such a coherent picture in the opposite direction. The consequences of large cardinals, at the arithmetic level (and more, as we climb up through the hierarchy) are compatible. The arithmetic consequences of any natural extension of $\mathsf{ZFC}$ fall somewhere within this hierarchy (as far as the theories we can currently analyze), even if the theory does not mention large cardinals. In fact, determinacy statements, incompatible with choice, also fall within this hierarchy and are mutually interpretable with large cardinals (again, as far as those theories we can currently analyze). This deep connections with determinacy are behind what we now call descriptive inner model theory, see

Grigor Sargsyan. Descriptive inner model theory, Bull. Symbolic Logic, 19 (1), (2013), 1-55.

Large cardinals provide us with generic absoluteness, and generic absoluteness, a natural requirement if we are interested in understanding the projective theory of the reals, requires the consistency of large cardinals. See this answer for a bit more on this issue; let me emphasize that this is not some technical or artificial requirement, but rather a natural extension of basic results in classical descriptive set theory.

Large cardinals seem inherently necessary to mathematical practice, not just set theory. Harvey Friedman has written extensively on this issue.

In short: We have a very clear measure of progress understanding large cardinals and their consequences. By this measure, we can now understand many set theoretical issues that do not involve large cardinals but for which they are necessary in deeper ways (not just consistency-wise). This measure actually requires the large cardinals, we do not have anything like that without them. This measure is meaningful even in settings that are not set theoretical, and seems unavoidable even within mathematical practice (though it is perhaps too soon to tell how significant this will be at the end for "practicing mathematicians"). We do not have any serious mathematical model where large cardinals would be inconsistent, however, we have a serious program of research that would ultimately teach us that, were this the case. The program has provided us, instead, with many positive results (in particular, we have nice inner models for measurability, for strong cardinals, for Woodin cardinals, and we have nice inner models of models of determinacy, that capture the large cardinals that provide us with the consistency of the determinacy statements).

To conclude, we understand (motivate/explain) large cardinals within the larger context of reflection principles, the simplest of which follow already from $\mathsf{ZFC}$. (So, we have a natural generating principle for them.) On the other hand, I know of no objections to large cardinals beyond "they are too large" or "they do not feel right", neither of which seems mathematical to me. The first also seems particularly artificial.

The only 'program' towards their inconsistency (that I am aware of) instead produced many interesting consequences for the partition calculus at the level of $0^\sharp$ (and is perhaps responsible for the early theory of $0^\sharp$ itself). As far as I understand, a similar attempt to disprove measurable cardinals resulted instead in the development of the covering lemma, which has since been one of the key tools to measure our understanding of particular large cardinals as part of the inner model program, see

William J. Mitchell. The covering lemma. In Handbook of set theory. Vols. 1, 2, 3, Matthew Foreman, and Akihiro Kanamori, eds., pp. 1497–1594, Springer, Dordrecht, 2010. MR2768697. (Wayback Machine)

Perhaps I should add that our intuitions about large cardinals do not come for free, but are the result of the programs mentioned above. I am in particular suspicious of a priori mistrust of large cardinals, since it tends to hide misunderstanding, or ignorance, of the actual mathematics involved in these programs.

Answer 2

Most of the answers have addressed the "consistency" part of the original question, "Why is it so unreasonable to think that the existence of large cardinals contradicts ZFC?" There's another part, about existence, "Are there any mathematicians that do belive that large cardinals don't exist?" I'd like to point out that belief in the existence of large cardinals and belief in their consistency are not necessarily the same thing. In the first place, someone who accepts the real existence of the natural numbers but not of fancier sets might well believe that large cardinal axioms are consistent with ZFC (that's an arithmetical belief), but not believe in the existence of large cardinals or even of small cardinals like $\aleph_1$. In the second place, even a Platonist might believe the consistency of some axioms for reasons other than a belief in their truth. For example, I consider it very likely that large cardinal axioms are consistent (for reasons others have explained in their answers) up through the cardinals for which there is a clear (to the experts) inner model theory, and even that these axioms hold in certain proper-class size inner models. On the other hand, I've never been able to persuade myself of the actual existence of even subtle cardinals (which are so small that they're consistent with $V=L$). My difficulty here is that the definition of "subtle" (like that of "measurable" and many others) seems to include a combinatorial aspect quite different from mere largeness (though of course the definitions imply a certain largeness). I'm generally OK with mere largeness, but I don't see why there should really exist cardinals with these additional combinatorial properties. (A plausible-looking picture of the real world might be that there really aren't any measurable cardinals, but that all singular cardinals are measurable (and more) in some inner models.)

Answer 3

First of all, why is it so unreasonable to think that ZFC itself is contradictory? Because we have a good intuition about sets and we have a lot of experience with ZFC. The same basically applies to large cardinals. What I am trying to point out here is that assuming large cardinals is not much more unreasonable than passing from Peano Arithmetic to ZFC. (Edit: David Roberts points this out is his answer: If you doubt the consistency of large cardinals, why not start earlier and question the existence of the set of natural numbers?)

A lot of work has been done on the subject of large cardinals and besides Reinhardt cardinals, nothing has ever turned out to be inconsistent.
There is the so-called inner model program where one assumes the existence of a certain large cardinal and tries to build an (easily controllable) smallest model of set theory in which there is such a large cardinal and which contains all the ordinals.
The idea is that because we have a good understanding of the final inner model, we would notice during the construction of the model if there were any problems with the consistency of the large cardinal in question.

This program has worked out so far to rather high levels of the hierarchy of large cardinals.

Another important point to believe in the consistency of large cardinals is the fact that the consistency strengths of large cardinals are apparently linearly ordered.
In other words, it has not yet happened that there is a natural notion of a large cardinal which cannot be compared to the other known large cardinals in terms of consistency strength (a large cardinal is stronger that the other if the consistency of the first implies the consistency of the second). That indicates that there is a natural direction in which set theory can be strengthened, which is a remarkable (heuristic) observation.
(For certain types of large cardinal axioms it can actually be proved that they form a linearly ordered hierarchy.) I find it unlikely that we should see this linearly ordered structure above a large cardinal whose existence is actually inconsistent.

So, if there is a natural direction to strengthen our basic theory, why not go all the way and work in the strongest theory in that direction, by assuming the existence of all (consistent) large cardinals. Among other things, it has turned out that the existence of large cardinals implies a rather nice structure theory for the subsets of the real line that are definable is a certain sense (projective sets).
And as pointed out above, there are good reasons to believe in the consistency of large cardinals.

Concerning arguments against large cardinals, I would think that the main objection is that large cardinals have no effect on ordinary mathematics. But as I have pointed out in the previous paragraph, this is not entirely true. Moreover, even though this is cumbersome, most of "ordinary mathematics" can actually be carried out in weak systems of number theory. In particular, the full strength of ZFC is unnecessary most of the time.

Answer 4

You asked:

Why is it so unreasonable to think that the existence of large cardinals contradicts ZFC?

It's not "unreasonable," any more than it's "unreasonable" to disbelieve the Riemann Hypothesis. If you systematically poll people and ask them at what point in the hierarchy do they start developing acrophobia (i.e., doubting that infinities of that "size" exist), you'll get answers all along the spectrum. The doubts, however, are usually of the form, "I don't feel that there's enough evidence to make me believe in them," rather than any concrete arguments against their existence. Once in a while you may run into someone who believes $V=L$ strongly enough to use that as a reason to disbelieve in measurable cardinals, but that's pretty rare.

What you're observing in the literature is that large cardinals lead to a very fruitful and insightful theory. Assuming that they don't exist, on the other hand, doesn't seem to lead to very interesting results. So that's why the literature is biased in this direction. Similarly, you'll find lots of papers that assume the Riemann Hypothesis or that assume $P \ne NP$, and build up a complicated conjectural picture of the mathematical universe on that basis, even though it's theoretically possible that the whole picture could collapse like a house of cards if the underlying assumption were shown to be false. The reason is that the picture is a compelling one that "feels true" and that generates many ideas and corollaries that can sometimes be verified unconditionally. Assuming the opposite hypotheses, however, rarely leads anywhere—though in the cases where it does lead somewhere, people don't hesitate to mention it.

Answer 5

Nelson (and at the linked question, we see also Doyle and Conway) is famous for not believing in the existence of $\aleph_0$, which is the cardinal of a limit ordinal (i.e. $\omega$), so can be considered large, but not large from a usual set theory point of view. He would be called a finitist in this respect. More precisely, his axioms of arithmetic do not presuppose the existence of a natural numbers object, and do not show it either.

Edit: In fact Nelson is an ultrafinitist, in that he doubts the existence of large natural numbers, and gives a combinatorial example (see my comment below) of a number the finiteness of which he questions (this corrects a mis-statement on my behalf on the original version of the question, where I called Nelson an ultrafinitist for not 'believing in' $\aleph_0$).

Answer 6

Large cardinals offer a detailed coherent picture — with a single principle, that of symmetry, reaching even (essentially) the strongest large cardinals. They continually offer new results — without inconsistency. For weaker large cardinals (up through Woodin cardinals) we have canonical well-understood fine-structural models, with a variety of intuitively true (or otherwise well-analyzed) seemingly unrelated combinatorial principles about real numbers and other sets implying their consistency. And while we are not there yet, we expect many more such results at the level of supercompact cardinals.

However, per the question title, the rest of the answer will be arguments against large cardinals, which are important both because of genuine doubts (especially about certain things) and for the insight the arguments offer. Many of the arguments also apply to ZFC, but become stronger as the large cardinal strength increases.

The arguments separate into:
• Arguments for inconsistency of large cardinals.
• Arguments for non-existence of large cardinals (even if they are consistent).
• Arguments for not accepting large cardinals as axioms.

Moreover regarding truth/consistency, we can approximately separate mere doubts from affirmative arguments for falsehood/inconsistency, with most arguments being the former.

Arguments against acceptance as axioms:
• Doubts about large cardinal axioms (see the other paragraphs).
• The importance for the foundations of mathematics to be free of unnecessary doubt.
• The unfortunate general disinterest in foundations. Most mathematicians do not even know exactly what ZFC is. (This is relevant here because 'acceptance' has a sociological component.)
• Related to the above, the limited need to go beyond ZFC for results most mathematicians care about.
• Knowing that something is provable in a weak theory (or just ZFC) often gives us important information that goes beyond knowing that it is true.
• One can still use large cardinal axioms by including them as assumptions.

Doubts about consistency:
• Doubt about metaphysical existence of infinite sets, or if they exist, about intuitive reasoning about such sets. Many believe that the observable physical universe is finite and/or that the operation of physical laws is computable, and that humans do not have privileged access to truth. Also, some facts that are obviously true about finite sets fail for the infinite.
• Lack of inner model and core model theory beyond Woodin cardinals. While (as set theorist John Steel put) "all roads lead to projective determinacy", we do not yet know whether all roads lead to analogs of supercompact cardinals.
  - Core model induction has not yet reached a Woodin limit of Woodin cardinals (but there is progress). Core models (including core model induction) are our main method of showing that natural plausible combinatorial propositions at low levels of $V$ imply consistency of large cardinals.
  - Existence of canonical inner models for a stationary set of Woodin cardinals is still open as of 2018.
  - While under plausible conjectures, current definitions of fine-structural models reach subcompact cardinals (and slightly further), supercompact cardinals show a qualitively different and anti-core-model behavior, such as consistency of indestructibility.
• Lack of sufficient heuristic 'proofs' of consistency of ZFC:
  - It is hard to find compelling arguments for consistency of ZFC that make no mention of infinity (other than the argument that consequences of ZFC have been well-understood with no inconsistency found; also, "hard" need not mean "impossible").
  - Lack of enough natural true arithmetic statements that are known to imply consistency of ZFC.
  - Lack of reasonable ordinal notation systems that have been proved to reach ZFC (or even just $\mathrm{Z}_2$). Ordinal notation systems (and to a lesser extent, canonical inner models) give us a well-understood picture of how a theory operates, which (among many other benefits) counteracts doubts about consistency.

Affirmative historical arguments for inconsistency:
• Historically, paradoxes (including Russell paradox) were used to argue against infinity (including consistency of infinity) but these arguments have receded, as ZFC has no shown sign of being inconsistent through such a paradox.
• Kunen's inconsistency was used to argue against superficially similar axioms: Since existence of nontrivial elementary $j:V_{λ+2}→V_{λ+2}$ is inconsistent, why should using $V_{λ+1}$ be different? However, the current understanding is that there is no inconsistency without the axiom of choice (but see below), and unlike $V_{λ+2}$, choice does not cause problems for $V_{λ+1}$ and (starting from a stronger axiom for ZF) we can get choice in a generic extension.
• More recently, Hugh Woodin has argued (see his work on the HOD dichotomy) that Reinhardt cardinals for ZF are inconsistent. Specifically, for cardinals such as measurable (or even strong), there are canonical ordinal definable inner models that use the restrictions of the extenders from the embeddings on $V$ (and get the same large cardinals from these extenders), but he was able to show that modulo consistency of a strengthening of ZF + Reinhardt cardinal (that is weaker than Berkeley cardinals), this need not hold for supercompact cardinals. Personally I think that this failure is why supercompacts have been so useful for consistency of combinatorial propositions — unlike weaker cardinals, the embeddings (in a sense) capture a large part of $V$, and thus cannot be generally included into canonical inner models as is — but there is still much that we do not understand.

In between consistency and existence, we have various degrees of soundness. Properties inconsistent with the axiom of choice can still hold at those levels. Thus, for example, near the top of the known hierarchy (and thus, with the highest doubts about consistency), we have the $Π^V_2$ statement "For every cardinal $κ$, there is a model of 'ZF + Berkeley cardinal' closed under $κ$ sequences." The truthfulness of such statements is generally regarded as closely related to consistency. However, this belief assumes a fundamental soundness of the large cardinal hierarchy, which has strong analytical and empirical support, but can also be argued against (for example, see the arguments for $V=L$).

Doubts on existence of large cardinals:
• Formalists and adherents to various related philosophies believe that infinite (or failing that, uncountable) sets do not actually exist. What may exist are finite quasimodels (or countable models) of various set axioms, but they hold that there is no single preferred truth predicate for $(V,∈)$. The lack of consensus on CH is often cited to support this. Various large cardinal axioms may hold in some but not other theories/quasimodels/models, and the use of ZFC is a useful common convention. For them, existence of large cardinals morphs into goodness of using ZFC + A (for various A) for mathematical work.
• Consistency of large cardinals is fundamentally different from existence. To fully accept inaccessible cardinals, a set platonist would need to know that they metaphysically exist, or for a symmetry platonist (a type of truth-value platonist), that we have enough symmetry that truth values of statements at the level of inaccessible cardinals are unambiguous.
• The replacement axiom schema (and choice) may intuitively follow from "For every ordinal $α$, a process that can be always be continued can be iterated $α$ times", but this reasoning is insufficient to reach inaccessible cardinals.
• By Kunen's inconsistency, some otherwise natural large cardinal axioms are inconsistent with choice. Perhaps we will discover a stronger natural true combinatorial principle that refutes more large cardinal axioms.
• A canonical theory for a given expressiveness level may require consistency strength that on its face corresponds to a higher level. For example, a true reasonably complete axiomatization of second order arithmetic requires projective determinacy, and projective determinacy can be ascertained by just studying real numbers (disclaimer: not everyone agrees with this), which in turn gives consistency of Woodin cardinals. In this manner, doubt about consistency of Woodin cardinals may translate into doubt about existence of the set or class of all real numbers (and quantification over it). We do not know how much strength is required for third order arithmetic and higher expressiveness levels, and a breakdown would argue for non-existence of higher levels.
• Large cardinals can be destroyed by forcing. For example, we can destroy all measurables by Prikry forcing (making them cofinality $ω$ without changing other cofinalities), and it is arguably unclear why the ground model rather than the extension is a better analog to $V$. Also, by adding clubs, starting with a model of GCH, I think we can destroy all Mahlo cardinals while preserving GCH and cofinalities; and no forcing can resurrect Mahloness.

Affirmative arguments for non-existence:
• Predicativist argument for $V=L$. Predicativists hold that certain objects are (essentially) constructed in stages, and that quantification is fully permitted only over the previously constructed stages. Most predicativists start with only natural numbers, but if one also accepts arbitrary ordinals, the result is the constructible universe $L$ — each stage of $L$ is added predicatively. One would then object to zero sharp analogously to the traditional predicativist objection to zero hyperjump (Kleene's $\mathcal{O}$). Also, some mathematicians simply like $L$ as a canonical tidy universe that satisfies ZFC (and is $Σ^1_2$-correct).
• Against measurable cardinals:
  - The nonrigidity (nontrivial elementary embedding $V→M$) arguably qualitatively alters the behavior of $V$ and is contrary to how $V$ behaves at lower levels.
  - Measurable cardinals cause a few counterexamples in topology.
  - Under $HOD(ℝ)⊨\mathrm{AD}$, $ω_1^V$ is the least measurable in HOD (assuming the proof of lack of lower measurables applies here). The absence of lower measurable cardinals, combined with the fact that every ordinal definable subset of $ω_1$ is constructible from a real, and that the measure on $ω_1$ is 'caused' by an external structure (the club filter on $ω_1$) (along with the general largeness of measurables and existence of Prikry forcing) suggest a possibility that measurable cardinals do not occur in the cumulative hierarchy on their own, but reflect a restriction on $\mathcal{P}(κ)$.
(However, on the balance, I find that evidence supports existence of measurables.)

Answer 7

According to this comment by François Dorais, Petr Vopěnka doubted the largest of large cardinal axioms, and gave an example of a statement he thought was false, yet was implied by some large cardinal axioms. No one has ever found a proof that his statement -- now known as Vopěnka's principle -- is false, and now it is regarded as just another large cardinal axiom.

There's more detail in the thread, but I'll give a quick summary: A set or class of graphs is "rigid" if there exists no nontrivial graph homomorphisms between graphs in the set. Vopěnka proved that there exist sets of rigid graphs of arbitrarily large cardinality, and conjectured that there must be a proper class of graphs with the same property. If anyone constructs such a class, then that proves that the largest of the large cardinal axioms are inconsistent with ZFC.

Answer 8

I have not yet discussed these concerns about the consistency of I3-I0 cardinals with many set theorists. Please upvote if you think my concerns about the algebra of elementary embeddings should produce at least some doubts about the consistency of rank-into-rank cardinals. Please downvote if you do not think my concerns about the algebras of elementary embeddings should produce doubts about the consistency of rank-into-rank cardinals. I have made this post community wiki so that people will feel more free to upvote or downvote this answer based on these new criteria.

I personally have some doubts about the consistency of rank-into-rank cardinals since I am concerned about a possible inconsistency arising from the algebras of rank-into-rank embeddings.

Define the Fibonacci terms $t_{n}$ for $n\in\omega$ by $t_{1}(x,y)=y,t_{2}(x,y)=x$ and $t_{n+2}(x,y)=t_{n+1}(x,y)*t_{n}(x,y)$. A reduced permutative LD-system [1] is a left-distributive algebra $(X,*)$ together with an element $1\in X$ such that $1*x=x$ for each $x\in X$ and where for each $x,y\in X$ there exists an $n$ with $t_{n}(x,y)=1$.

Let $\mathcal{E}_{\lambda}$ be the set of all elementary embeddings $j:V_{\lambda}\rightarrow V_{\lambda}$. Then define $j*k=\bigcup_{\alpha<\lambda}j(k|_{V_{\alpha}})$ for $j,k\in\mathcal{E}_{\lambda}$. If $\gamma<\lambda$ is a limit ordinal, then define $j\equiv^{\gamma}k$ if $j(x)\cap V_{\gamma}=k(x)\cap V_{\gamma}$ for each $x\in V_{\gamma}$ ($\equiv^{\gamma}$ is a congruence on $(\mathcal{E}_{\lambda},*,\circ)$)

Intuitively, the reduced permutative LD-systems resemble the algebras of elementary embeddings $(\mathcal{E}_{\lambda}/\equiv^{\gamma},*,\circ)$ in the following ways:

1. The reduced permutative LD-systems have a notion of a composition operation. In particular, if $x,y\in X$, then we can define $x\circ y=t_{n+1}(x,y)$ where $n$ is a natural number such that $t_{n}(x,y)=1$.

2. The reduced permutative LD-systems have a notion of a critical point. We define $\textrm{crit}(x)\leq \textrm{crit}(y)$ if there exists some $n$ where $x^{n}*y=1$. The notion of a critical point is very well behaved for reduced permutative algebras since critical points in reduced permutative LD-systems satisfy most of the main properties that critical points of rank-into-rank embeddings satisfy. Furthermore, the notion of a critical point in reduced permutative LD-systems is not just a generalization from set theory to algebra but the notion of a critical point in a reduced permutative LD-system is an essential part of the theory of permutative LD-systems.

3. Every algebra of the form $\mathcal{E}_{\lambda}/\equiv^{\gamma}$ is a reduced permutative LD-system.

The following results show that the notion of a critical point in a permutative LD-systems behaves nearly the same as the notion of a critical point in an algebra of elementary embeddings.

$\mathbf{Theorem:}$ Suppose $X$ is a reduced permutative LD-system.

1. $\textrm{crit}[X]=\{\textrm{crit}(x)|x\in X\}$ is a linear ordering.

2. If $\textrm{crit}(x)\leq\textrm{crit}(y)$, then $\textrm{crit}(r*x)\leq \textrm{crit}(r*y)$.

3. $\textrm{crit}(x\circ y)=\min(\textrm{crit}(x),\textrm{crit}(y))$

4. $x=1$ if and only if $\textrm{crit}(x)$ is the largest element in $\textrm{crit}[X]$.

If $x\in X$, then define a mapping $x^{\sharp}:\textrm{crit}[X]\rightarrow \textrm{crit}[X]$ by letting $x^{\sharp}(\textrm{crit}(y))=\textrm{crit}(x*y)$.

$\mathbf{Theorem:}$ Suppose $X$ is a reduced permutative LD-system.

1. $\alpha\leq x^{\sharp}(\alpha)$

2. $\alpha<x^{\sharp}(\alpha)$ precisely when $\textrm{crit}(x)\leq\alpha<\max(\textrm{crit}[X])$

3. The restricted mapping $x^{\sharp}|_{A}$ where $A=\{\alpha\in \textrm{crit}[X]|x^{\sharp}(\alpha)<\max(\textrm{crit}[X])\}$ is injective.

4. $x^{\sharp}(y^{\sharp}(\alpha))=(x\circ y)^{\sharp}(\alpha)$

At this point, since permutative LD-systems and algebras of elementary embeddings are so similar, it is reasonable to conjecture that every finite reduced permutative LD-system is isomorphic to some subalgebra of some $\mathcal{E}_{\lambda}/\equiv^{\gamma}$. This conjecture is false. Consider the following facts.

$\mathbf{Fact:}$If $j:V_{\lambda}\rightarrow V_{\lambda}$ is elementary, then $j*j(\alpha)\leq j(\alpha)$ for all $\alpha<\lambda$. In particular, $\textrm{crit}((j*j)*k)=(j*j)(\textrm{crit}(k))\leq j(\textrm{crit}(k))=\textrm{crit}(j*k)$ for all $j,k\in\mathcal{E}_{\lambda}$

On the other hand, there exists permutative LD-systems $(M,*)$ along with $x,y\in M$ such that $\textrm{crit}((x*x)*y)>\textrm{crit}(x*y)$ (such algebras $(M,*)$ were discovered using computer calculations). Therefore, the algebra $(M,*)$ cannot arise from the algebras of elementary embeddings. One possible explanation between this discrepancy is that may be possible to show that $(M,*)$ is actually a subalgebra of some $\mathcal{E}_{\lambda}/\equiv^{\gamma}$ in some model and thus obtain an inconsistency.

The algebras $M$ where $\textrm{crit}((x*x)*y)>\textrm{crit}(x*y)$ for some $x,y\in M$ together with the great consistency strength of rank-into-rank cardinals has given me some doubts about the existence and consistency of rank-into-rank cardinals. After all, the rank-into-rank cardinals are very close to the Kunen inconsistency, and they are far above the cardinals for which there exists a good inner model theory. Furthermore, the mere fact that rank-into-rank cardinals may be used to prove purely algebraic results is a reason to believe that rank-into-rank cardinals are the most vulnerable spot of the large cardinal hierarchy to an inconsistency.

With everything being said, there is likely a better explanation for the existence of reduced permutative LD-systems $(M,*)$ with $\textrm{crit}((x*x)*y)>\textrm{crit}(x*y)$. It is likely that the only reason there seems to be a discrepancy between algebra and set theory is that the algebras of rank-into-rank embeddings are very poorly understood. When the algebras of rank-into-rank embeddings become better understood, I will likely recant my doubts about the existence and consistency of rank-into-rank cardinals. Lastly, the near inconsistency of rank-into-rank cardinals seems to imply that algebras of rank-into-rank embeddings may be used to continue to prove new good results about algebraic structures which do not have proofs in ZFC. I therefore think it would be wise to search for a possible inconsistency of rank-into-rank cardinals so that when no inconsistency arises, plenty of algebraic results remain.

If there is an inconsistency arising from the algebras of elementary embeddings, then one can probably show that $n$-huge cardinals are inconsistent as well for fairly small $n$. On the other hand, the huge cardinals are probably safe from such an inconsistency.

I should mention that others in the set theory community have not expressed these doubts since the algebras with $\textrm{crit}((x*x)*y)>\textrm{crit}(x*y)$ are very new and no one else is working on them.

[1] Generalizations of Laver tables, Joseph Van Name (in progress; hopefully almost ready for Arxiv)

[2] http://boolesrings.org/jvanname/2016/04/05/set-theory-seminar-february-19-2016-generalized-laver-tables-part-ii/

Answer 9

Erinna, in your question you use the word 'exist'. In the philosophy of mathematics, that is a word that is part of discussion. If you follow a Plato philosophy, then there is a perfect mathematical universe, were all the objects 'exists'. Then you can talk about the existence of large cardinals. If you remove those objects from that universe, then that is probably considered a loss by many.

However, such philosophy has problems. The main problem is how this mathematical universe interacts with our daily world. Without such interaction, we can not access such universe. Of course, I have some personal opinions here, and one can have a long discussion.

In some philosophies one tries to have a more limited mathematics, with only mathematical objects with a clear meaning. In such philosophy, (large) cardinals are not part of that limited mathematics. However, that does not mean that other constructs are entirely rejected. They can still be mental constructs, or meta-mathematics. Constructs that can be used to do mathematics that has more clear meaning.

As said, with philosophy you can have lot of discussion and there are many views.

I think a better question is whether '(large) cardinals must be part of the fundamentals of mathematics'. I strongly say no to that question, although I do not object against (large) cardinals as mental construct.

Lucas

Answer 10

"Why is it so unreasonable to think that the existence of large cardinals contradicts ZFC?"

I don't think that it is so unreasonable, given the Kunen inconsistency, for example. No one suspected that naive set theory, which was very productive for many years, was inconsistent, until Zermelo and Russell revealed "Russell's paradox." Consistency is a delicate thing, and that's why mathematicians prefer epistemological safety over ontological wealth. "The idea of paraconsistency is that coherence is possible even without consistency. Put another way, a paraconsistent logician can say that a theory is inconsistent without meaning that the theory is incoherent, or absurd." https://iep.utm.edu/para-log/ So, even if various theories positing the existence of various large cardinals are useful and interesting to set theorists, mainstream mathematicians need not study them, and probably ought not accept the "truth" of the theorems of such theories, including $V \neq L$, in their day-to-day work. "Agnosticism" regarding the existence of large cardinals (rather than "theism" or "hard atheism") is probably the wisest choice for the mainstream mathematician.